1. Field of the Invention
This invention relates to a multi-band radio receiver employing a phase locked loop, and more particularly, to a multi band radio receiver in which coarse tuning is accomplished by disabling a phase locked loop and adjusting a coarse tuning reactance, deriving from the frequency generated during coarse tuning a scale factor and employing the scale factor in the phase locked loop during normal operation to control the frequency.
2. Description of the Prior Art
Prior art multi band radio receivers employ variable inductance and/or capacitance elements which are mechanically varied to adjust the oscillating frequency of a local oscillator to tune an rf signal. In receivers having a wide tuning range, for example, from 0.1-30 MHz, problems of stability and band spread exist which are not satisfactorily solved in the prior art.
Stability problems in prior art devices arise principally due to the necessary use of inductance and capacitance elements in frequency determining components. Minor vibration or flexing of the receiver structure is sufficient to vary the reactance of these elements which result in variations in the local oscillator frequency and consequently the frequency to which the receiver is tuned. In addition, frequency determining components of this type tend to drift with temperature and age thereby providing an additional source of instability.
Band spread problems occur due to the wide frequency band which multi band receivers must tune. For example, at low frequencies a relatively large tuning adjustment of a variable capacitance or inductance is required to change the frequency a given amount. At high frequencies, the size of the tuning adjustment required between stations decreases drastically. The amount of tuning adjustment decreases so much, in fact, that it becomes virtually impossible to accurately tune a station at high frequencies using a control that is also satisfactory for low frequencies. In response to this problem, prior art multi band receivers are provided with a coarse tuning control and a fine or band spread tuning control. The coarse tuning control is used to set the reactance of a relatively large inductor or capacitor to control the tuning at low frequencies but merely to establish the general tuning region at high frequencies. The fine tuning control employs a relatively small variable reactance which varies the tuning frequency a relatively small amount about the general tuning region established by the coarse tuning reactance.
In an attempt to achieve higher frequency stability, phase locked loops have been employed to control the local oscillator of prior art receivers. However, such phase locked loops have a limited tuning range of, for example, about 10 KHz. Thus a medium or short-wave band receiver employing such a phase locked loop requires phase locking at increments of for example 10 KHz. This then requires a very large number of locked frequencies and results in a complicated and high cost receiver which is difficult to use.